An apparatus (a flow cytometer or a cell sorter) has been proposed which makes a liquid containing stained biological particles (minute objects to be measured: samples) flow through a flow path of a flow-path-formed member (flow cell), emits light from an illuminating unit (light emitting unit) to the biological particle, and detects scattered light or fluorescence from the biological particle using a detecting unit (light receiving unit), thereby obtaining the biological information of the biological particle (for example, see Patent Document 1).
In the related art, in the above-mentioned apparatus, a sample flow, which is a stream of the liquid flowing through the flow path of the flow cell, is surrounded by a sheath flow and is adjusted so as to flow in the vicinity of the center of the flow path. The light receiving unit is fixed such that the optical axis and focus of an optical system of the light receiving unit are aligned with the center of flow path in the flow cell on the plane which is substantially orthogonal to the sample flow. In addition, the light emitting unit emits light so as to be focused on the sample flow and receives scattered light or fluorescence from the sample. In this way, it is possible to obtain scattered light or fluorescence with high sensitivity or obtain the optical information of the sample with a small variation. Therefore, it is possible to optimally adjust the position of the optical axis of the light emitting unit with respect to the sample flow while actually emitting light to the sample flow to check the optical information.